The present invention relates generally to aircraft navigation and landing. More specifically, embodiments of the invention relate to methods and systems for navigating and landing aircraft.
A VHF (very high frequency) Omni-directional Range (VOR) navigation system is implemented by dispersing VOR transmitter facilities across a geographic area. VOR receivers are located on aircraft which navigate through such a geographic area. The basic principle of operation of the VOR navigation system includes transmission from the VOR transmitter facilities transmitting two signals at the same time. One VOR signal is transmitted constantly in all directions, while the other is rotatably transmitted about the VOR transmission facility. The airborne VOR receiver receives both signals, analyzes a phase difference between the two signals, and interprets the result as a radial to or from the VOR transmitter 100. The VOR navigation system allows a pilot to simply, accurately, and without ambiguity navigate from VOR transmitter facility to VOR transmitter facility. Each VOR transmission facility operates at a frequency that is different from surrounding VOR transmitters. Therefore a pilot can tune their VOR receiver to the VOR transmission facility to which they wish to navigate.
Widely introduced in the 1950s, VOR remains one of the primary navigation systems used in aircraft navigation.
The rotating transmission signal is achieved through use of a phased array antenna at the VOR transmission facility. Separation between elements of the array causes nulls in the signal received at the aircraft. Element separation may also cause erratic signal reception when an aircraft is within an area above the antenna array. Such nulls result in a conically shaped area originated at the VOR transmitter and extending upward and outward at a known angle. The conically shaped area is sometimes referred to as a cone of confusion. When an aircraft is within the cone of confusion, a pilot typically navigates utilizing only heading information, a process sometimes referred to as dead-reckoning. It is advantageous for a pilot to know that he or she is entering the cone of confusion.
An instrument landing system (ILS) also includes ground based transmitters, located at runways, and airborne receivers. The ILS transmitters transmit signals, received by the receivers on the aircraft, which are utilized to align an aircraft's approach to a runway. Typically, an ILS consists of two portions, a localizer portion and a glide slope portion. The localizer portion is utilized to provide lateral guidance and includes a localizer transmitter located at the far end of the runway. The glide slope portion provides vertical guidance to a runway and includes a glide slope transmitter located at the approach end of the runway. More specifically, a localizer signal provides azimuth, or lateral, deviation information which is utilized in guiding the aircraft to the centerline of the runway. The localizer signal is similar to a VOR signal except that it provides radial information for only a single course, the runway heading. The localizer signal includes two modulated signals, and a null between the two signals is along the centerline path to the runway.
The glide slope provides vertical guidance to the aircraft during the ILS approach. The glide slope includes two modulated signals, with a null between the two signals being oriented along the glide path angle to the runway. If the aircraft is properly aligned with the glide slope signal, the aircraft should land in a touchdown area of the runway. A standard glide slope or glide path angle is three degrees from horizontal, downhill, to the approach-end of the runway. Known flight guidance systems, sometimes referred to as flight control systems, are configured to assume a nominal glide path angle, for example, three degrees. Some known flight guidance systems have difficulty capturing the null in the glide slope signal at runways whose glide path angle varies significantly from the assumed glide path angle.
The VOR, localizer, and glide slope all provide an angular deviation from a desired flight path. The angular deviation is the angle between the current flight path and the desired flight path. Depending on a distance from a transmitter, a linear change to the flight path to correct an angular deviation can vary widely. A linear deviation is the current distance between the current flight path and the desired flight path. Furthermore, most flight guidance systems are better suited to receive and process linear deviations from a desired flight path. Known flight guidance systems utilize data from distance measuring equipment (DME) and radar altimeters to convert angular deviations in one or more of VOR, localizer, and glide slope, into linear deviations that can be acted upon by a pilot or a flight guidance system. Therefore, aircraft not equipped with DME or a radar altimeter are not able to convert the angular deviations into linear deviations that can be optimally acted upon by the flight guidance system.
Known flight guidance systems utilize distance information from DME to estimate a distance to a VOR transmitter. The estimated distance, along with an angular deviation as determined from the VOR bearing is utilized to determine a linear deviation from a desired flight path and detect a cone of confusion. However, this approach assumes a default VOR transmitter station elevation, that the aircraft is equipped with DME, and that a DME station is co-located with the VOR transmitter.
Known flight guidance systems also utilize altitude information from, for example, a radar altimeter to estimate localizer deviations. The altitude, along with an angular deviation as determined by the localizer receiver is utilized along with an assumption of runway length to determine a localizer linear deviation from a desired flight path. For glide slope linear deviations, the altitude, an angular deviation as determined by a glide slope receiver, and an assumed glide path angle are utilized to estimate the linear deviation from a desired glide slope. These estimations assume that the aircraft is equipped with an altitude measuring device (e.g. radar altimeter). It would be advantageous to utilize actual data relating to VOR, localizers, glide slopes, and runway lengths and altitudes when providing a pilot or an auto pilot system navigation data. Similarly, it would be advantageous to provide such navigation data in aircraft which are not equipped with radar altimeters or DME.
In addition, known flight guidance systems are not able to properly capture the localizer signals under conditions of high ground speeds and high intercept angles, due to the limited beamwidth of the transmitter and saturation of the localizer receiver at the necessary aircraft positions. This results in late captures and potentially significant overshoot in acquiring the proper course.
Further, known flight guidance systems also do not track the selected VOR course while traversing the “cone of confusion”, depending on maintaining the aircraft heading at the time the cone of confusion is entered. This may result in significant tracking errors when the VOR signal is re-acquired upon exiting the cone, especially if wind changes or selected course occur during passage of the VOR station.